This application claims the priority of German Application 19923738.7, filed May 22, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system for supplying electrical apparatus with electrical energy in a vehicle powered by an internal combustion engine or by fuel cells, as well as a method for supplying power by means of such a fuel cell system in a vehicle.
Fuel cell systems for supplying electrical apparatus of a motor vehicle powered by an internal combustion engine are disclosed in German Patents DE 195 23 109 A1 and DE 197 03 171 A1. These patents use a fuel cell system instead of a generator driven by the internal combustion engine. Fuel cells convert the chemical energy released by the oxidation of a fuel to electrical energy, the fuels used being, for example, hydrogen, hydrocarbons or methanol, and the oxidizers being usually air or pure oxygen. In the fuel cell systems of the above patents, the hydrogen is obtained as fuel for the fuel cell system by the separation or reformation of the liquid fuel that is necessary for the operation of the internal combustion engine. The fuel components left over are used to power the internal combustion engine. The hydrogen thus obtained is fed as fuel to the anode side of the fuel cell system. Usually, atmospheric oxygen is used as the oxidizer, and it is aspirated from the ambient air by a compressor and fed to the cathode side of the fuel cell system.
The compressor used for delivering atmospheric oxygen to the fuel cell must in turn be powered electrically or mechanically, so that electrical or mechanical energy is additionally consumed, which lowers the efficiency of the power generation. Furthermore, such a compressor is an additional component liable to trouble.
One object of the present invention is to create a fuel cell system of the kind described above, which achieves a greater power generation efficiency while at the same time using fewer components. It is another object of provide a system which is able to be operated very effectively.
The fuel cell system of the invention is connected by a supply line with the compressed air system of the vehicle, which serves for supplying the compressed air brake system, among other things, in order to deliver the necessary atmospheric oxygen to the fuel cell system. Especially in the case of a vehicle powered by an internal combustion engine, in which the internal combustion engine drives the air compressor of the compressed-air system, the fuel cell system supplying the electric power is connected to the air compressor by a supply line for feeding the necessary atmospheric oxygen.
Usually the compressed air system of the vehicle operates at about 8 to 12 bar. The air compressor serving to supply this compressed air system in vehicles with an internal combustion engine is either driven directly or with a system of belts. The rate of air delivery depends on the speed of the motor. In order to make sufficient air available for the brake system, even at low speeds, the air compressor must be of an appropriate size, and also intermediate tanks must be provided for the compressed air (e.g., when the motor is idle). Comparable conditions prevail in vehicles driven by fuel cells. As a result, at high rpm as well as in long travel distances in which little braking is done, the air compressor compresses and delivers great amounts of air and most of it is unused excess air which is discharged into the exhaust system of the vehicle.
The invention now permits the air compressed and delivered by the compressed air brake system to be used effectively for the generation of power by a fuel cell system, so that the efficiency of the power generation is increased and there is no longer a need for a compressor previously required for ambient air for the fuel cells.
The invention is not limited to a hydrogen fuel cell with a hydrogen tank of its own, but is suitable for any fuel cell systems which are used in motor vehicles to produce electric current, such as those which produce the necessary hydrogen by separating or reforming a hydrocarbon or alcohol, or which have a liquid anode circuit like direct methanol fuel cells (DMFC).
The invention can further be used both in vehicles with an internal combustion engine as well as an electric drive, as long as the vehicle has a compressed air system.
It is advantageous to reserve a portion of the compressed air produced by the compressed air system and to connect the reserve tank to the supply line of the fuel cell system for atomospheric oxygen. In this manner a briefly elevated electric energy demand can be covered to the extent that the amount of fuel needed for the fuel cell system is available. It is practical to use also the tank that is present anyway in the compressed air to be delivered to the fuel cell system.
It can furthermore be advantageous to provide a compressor for ambient air, which can be connected to the supply line carrying atmospheric oxygen, and which is driven by this system. In this way, in the event of a stalled motor or lack of air in the brake system, the air supply to the fuel cells can be sustained independently of the internal combustion engine. The air necessary for starting the fuel cells can in this case be taken from a compressed air storage tank. The compressor of the present invention can be made smaller than the previously needed compressor which was alone responsible for feeding air to the fuel cell system.
The air compressor of the present invention may be electrically powered by the fuel cell system and at the same time feed the necessary atmospheric oxygen to the fuel cell system.
Before compressed air is introduced at the anode end of the fuel cells the pressure must be adapted for example by a pressure reducer. The use of reserve compressed air is possible in the case of low engine speeds, when the motor is off or in case of lack of compressed air in the compressed air system. The additional supplying of air to the fuel cell system by a compressor is especially advantageous in the case of high energy demand, e.g., due to the switching on of an air conditioner. Moreover, in this manner the air requirements of the fuel cells can be provided also when the vehicle is standing with electrical accessories running.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.